Perinatal lethality of Par4^–/– mice delivered by primiparous dams reveals spontaneous bleeding in mice without platelet thrombin receptor function

Protease-activated receptor 4 (PAR4) is a cell surface G protein-coupled receptor for serine proteases, such as thrombin. Par4^–/– mice have platelets that are unresponsive to thrombin and thereby allow examination of the importance of thrombin-induced platelet activation in (patho)physiology. Par4^–/– mice are protected against arterial thrombosis but show no evidence of spontaneous bleeding. This contrasts with the bleeding experienced by mice with marked thrombocytopenia, such as those with genetic deficiency of the transcription factor, nuclear factor erythroid 2 (Nfe2^–/–), that have high rates of perinatal death due to hemorrhage. Given this discrepancy in spontaneous perinatal bleeding between mice without platelets and those without thrombin-induced platelet activation mechanisms, we examined in detail the immediate postnatal survival of Par4^–/– pups. We observed significant postpartum loss of Par4^–/– pups derived from Par4^+/– intercrosses that was restricted to a dam’s first litter; only 9% of surviving pups genotyped as Par4^–/– in first litters and this normalized from the second litter onward (26%). A similar perinatal lethality in pups delivered by primiparous dams occurred in mice lacking platelets (Nfe2^–/–; 10%) but not in those lacking fibrinogen (Fga^–/–; 26%). These data, provide the first evidence of spontaneous bleeding in Par4^–/– mice, suggest that a dam’s first litter provides a greater hemostatic challenge than subsequent litters, and uncovers an important role for platelets—and more specifically thrombin-induced platelet activation—in hemostasis during these more traumatic births.